1. Field of the Invention
The present invention relates generally to noise distortion discrimination circuitry for radio receivers. More specifically, the present invention relates to techniques for discriminating multipath and pulse noise distortions in a digital radio receiver for an automobile.
2. Description of the Related Art
At any given time, radio receivers may receive radio signals transmitted by numerous different stations. Radio receivers typically tune to the radio signals transmitted by a particular station and convert these radio signals into program information. The signals transmitted by stations may include AM audio signals, FM audio signals, and data information. With respect to audio information, radio receivers typically attempt to provide a high quality reproduction of the audio information transmitted by the selected station. In attempting to produce these high quality audio output signals, the radio receiver may to experience various events that can cause distortions in the audio signals being received, processed, and output by a radio receiver. These events include impulse noise distortions and multipath distortions.
Impulse noise distortions are distortion events that evidence themselves as brief periods of unstable amplitude and frequency spikes within the audio signal. For example, with an automobile radio receiver, impulse noise distortions often arise due to ignition of the automobile and due to turning on and off electrical components, such as for example windshield wipers, power windows, cigarette lighter, etc. Any of these activities may cause an electrical impulse that will create transient impulses at the antenna input or within the radio receiver circuitry. In addition, when using prior art circuitry for FM broadcast, data information, which typically resides at frequencies above audio signal information, may be incorrectly interpreted as impulse noise. Thus, if not properly discriminated, impulse noise distortions or falsely determined impulse noise distortions may significantly degrade audio performance.
Multipath distortions are distortion events that evidence themselves as brief periods of significantly reduced signal power. Multipath distortions typically occur when the signal power of a transmitted signal received at an antenna of the radio receiver is reduced by an out-of-phase version of the same transmitted signal that has traveled to the antenna along a different path. Multipath distortions may occur whether the radio receiver is stationary or mobile. If the transmitted signal reaches the receiver along two different paths such that one signal is out-of-phase with respect to the other, multipath distortions may occur. For a moving receiver, for example one positioned within a moving automobile, the movement of the automobile may also cause time-varying or intermittent out-of-phase signals to be received by the antenna. If not discriminated, multipath distortions may also significantly degrade audio performance.